1. Field of the Invention
The present invention relates to a system for controlling the transmission ratio of a transmission, and more particularly to a system for controlling the transmission ratio of a mechanically operated infinitely or steplessly variable transmission for automotive vehicles.
2. Description of the Prior Art
The transmission ratio control of the infinitely variable transmission has heretofore been accomplished by means of a transmission ratio ("ratio" hereinafter) control actuator upon which hydraulic ratio control pressure from a flow rate control valve is applied. The ratio control pressure is produced by a flow rate control valve which is differentially amplified by a force exerted on the displacement of a cam operatively connected with a throttle valve and a counteracting output pressure of a hydraulic governor driven by an output shaft of a transmission. It is very difficult to design such control system as the ratio control actuator is directly driven by the differentially amplified pressure from the flow rate control valve. In other words, it is very difficult to obtain on the one hand a high governor pressure sufficient to drive a spool of the flow control valve, on the other hand to provide a low cam displacement, to precisely determine the spring strength of the flow control valve and to provide a sufficiently long stroke. In addition, a large force to depress an acceleration is required by the long displacement of the cam. It is preferable to obtain a compensation control hydraulic pressure by pressing a pilot piston by means of an additional cam linked with a throttle valve if pressures for compensation such as differential compensation for throttle opening and compensation at starting are additionally applied to the ratio controlling pressure, i.e. actuator driving pressure. Still a greater force is required for depressing the acceleration pedal to provide compensation pressure in such case. Furthermore, the design of flow control valve becomes still more difficult if a well-balanced hydraulic pressure for the control of actuator would be generated by means of only one flow control valve into which a compensation control pressure is also input. Such drawbacks in the prior art make it also difficult to obtain a reliable ratio controlling system covering a wide transmission ratio range.